ARC-TSPEC-HOWTO

Overview

This document provides a guide to using the TripleSpec spectrograph on the ARC 3.5m with PypeIt. It covers the specific configurations, calibration procedures, and data reduction steps necessary.

TripleSpec is a fixed format spectrograph that records spectra in the near infrared. It records five spectral orders for a slit of length 43 arcsec.

External (truss) lamps are available for flat fielding and wavelength calibration, but wavelength calibration is generally achieved using sky lines in the near-IR. Near-IR flats are generally taken both with truss lights on and off, so that thermal contributions from the telescope can be subtracted.

Most near-IR observations are obtained with the object being “nodded” along the slit to allow for pixel-to-pixel sky subtraction. PypeIt has the ability to process difference images, but the user needs to manually set up the difference image pairing.

Setup

Since TripleSpec has a fixed configuration, there should only be one setup file per night.

Run pypeit_setup

You create PypeIt setup files using the pypeit_setup command, giving it the name of spectrograph and the directory with the raw data. You probably want to run this from a separate directory in which you want the reduced data to reside.

pypeit_setup -s arc_tspec -r PATH_TO_RAW_DATA/ --gui

The –gui argument will make this open a graphical interface from which you can inspect, edit, and/or save the different configurations. You can also run pypeit_setup without this argument and let it create the configuration files by itself without showing you the details.

pypeit_setup attempts to sort files into separate configurations, and automatically identify which files are to be used for pixelflats, illumflats, bias, arcs, tilts, standards, and science frames; see Setup for additional details.

Since ARC TripleSpec headers do not contain information about the truss lamps states (as of April 2026), pypeit_setup cannot determine which images are flats, and which flats have the lamps on and which have lamps off. As a result, you will need to manually identify these in the .pypeit file, either through the GUI or after the file is saved. For the illuminated flats, add illumflat,pixelflat,trace to the frametype column, and for the unilluminated flats, add the keyword lampoffflats to the frametype column.

You will also need to identify which frames to use for wavelength calibration (frametype arc, tilt); you probably just want to use a single science frame for this.

Finally, you will need to identify frame groups for difference imaging. See the PypeIt documentation on A-B image differencing.

Note that the default setup for Triplespec turns off cosmic ray detection since this seems to have issues over-detecting CRs. If you want to use this, you will need to enable CR detection using mask_cr = True, and you will likely need to play with the LAcosmic CR parameters; see Basic Image Processing.

PypeIt has MANY parameters that can be used to adjust the reduction; see User-level Parameters for details if you want/need to modify the default behavior.

Non-standard binning

The pipeline has not been tested for non-standard (non 1x1) binning. Please contact the developers if there are issues.

Main Run

Once the PypeIt Reduction File is ready, the main call is simply:

run_pypeit arc_tspec_A.pypeit -o

The -o indicates that any existing output files should be overwritten. As there are none, it is superfluous but we recommend (almost) always using it.

The PypeIt’s Core Data Reduction Executable and Workflow doc describes the process in some more detail.

Note that the PypeIt reduction process can take quite a while!

Inspecting Files

Calibrations

The first set are Calibrations.

Slit Edges

PypeIt will map the slit edges using the trace frames.

Wavelengths

One should inspect the PypeIt QA for the wavelength calibration. These are PNGs in the QA/PNG/ folder.

Note: there are multiple files generated for every order. When the reduction is complete, you may prefer to scan through them by opening the MF_[configuration].html file under QA/; note, however, that you may find other useful PNG files in the QA/PNG directory that are not listed in the MF_[configuration].html file.

The final wavelength solution is a two dimensional fit with pixel along the order one axis and the order itself being the second.

Remember, the default calibration is in vacuum wavelengths.

Spectra

The code will generate 2D and 1D spectra outputs. One per science frame, located in the Science/ folder.

One can inspect the one dimensional spectra with pypeit_show_1dspec.

Using that tool, you can examine the individual orders.